1. Field of the Invention
The present invention generally relates to a pulse signal transmitting circuit and, more particularly, to a pulse signal transmitting circuit used for a line build out (LBO) type balanced line drive circuit which generates a pulse voltage signal to be transmitted through a transmission cable. The LBO type balanced line drive circuit using the pulse signal transmitting circuit according to the present invention is suitable for a subscriber""s line terminal apparatus.
2. Description of the Related Art
FIG. 1 is a circuit diagram of a conventional pulse signal transmitting circuit. The pulse signal transmitting circuit shown in FIG.1 comprises data input terminals 1 and 2, switching transistors 3 and 4, a transformer 5 (N1:N2=1:1.3) and a filter circuit 6. A secondary side of the transformer 5 is connected to an ABAM cable (shieldless pair line) 7 so as to transmit an output of the pulse signal transmitting circuit to a load circuit 8.
The switching transistors 3 and 4 are push-pull connected to each other, and are connected to the transformer 5 as a load circuit. A power source (+5V) is connected to a middle point of a primary winding of the transformer 5.
The ABAM cable 7 has a frequency characteristic as shown in a graph of FIG. 1-(e). That is, a cable loss increases as a frequency of the signal transmitted through the ABAM cable 7 increases. Such a frequency characteristic is compensated for by pre-distorting a pulse signal to be transmitted through the ABAM cable on the transmitter side. FIG. 2 shows an example of a pulse template recommended for a North America DS-1 apparatus specified by the Bell specification with respect to a transmission cable such as the ABAM cable 7. The pulse template shown in FIG. 2 is characterized in that an overshoot 31 and an undershoot 32 of a pulse signal are allowed. In order to conform to the specification, the pulse signal transmitting circuit shown in FIG. 1 is provided with the filter circuit 6. The filter circuit 6 is connected in series to the power source of the pulse signal transmitting circuit.
When a positive pulse signal (FIG. 1-(a)) is input to the data input terminal 1, the positive pulse signal is provided to a base of the transistor 3. Accordingly, the transistor 3 is turned on, and a current flows in a primary winding of the transformer 5 in a direction D1. At this time, the filter circuit 6 connected to the power source attenuates a low-frequency component of the positive pulse signal. As a result, a high-frequency component of the current flowing in the direction D1 is enhanced. Accordingly, a positive pulse signal is generated in a secondary winding of the transformer 5. Similarly, when a positive pulse signal (FIG. 1-(b)) is input to the data input terminal 2, the positive pulse signal is provided to a base of the transistor 4. Accordingly, the transistor 4 is turned on, and a current flows in the primary winding of the transformer 5 in a direction D2 which is opposite to the direction D1. Similar to the current flowing in the direction D1, a high-frequency component of the current flowing in the direction D2 is enhanced by the filter circuit 6, and a negative pulse signal is generated in the secondary winding of the transformer 5.
Accordingly, when the positive pulse signal (FIG. 1-(a)) is input to the data input terminal 1 and the positive pulse signal (FIG. 1-(b)) is input to the data input terminal 2, a bipolar pulse signal shown in FIG. 1-(c) is generated in the secondary winding of the transformer 5 which bipolar pulse signal has an enhanced high-frequency component. The bipolar pulse signal generated in the transformer 5 is transmitted to the load circuit 8 through the ABAM cable 7. While the pulse signal is transmitted through the ABAM cable 7, the high-frequency component of the pulse signal is attenuated and a pulse signal shown in FIG. 1-(d) is obtained at the load circuit 8.
As mentioned above, the conventional circuit is provided with the filter circuit 6 so as to obtain the pulse signal having an enhanced high-frequency component as shown in FIG. 1-(c). However, there is a problem in that a power loss occurs in the filter circuit 6 since the filter circuit 6 attenuates a low-frequency component of the current input to the transformer 5.
It is a general object of the present invention to provide an improved and useful pulse signal transmitting circuit in which the above-mentioned problem is eliminated.
A more specific object of the present invention is to provide a pulse signal transmitting circuit which can generate a pulse signal having an enhanced high-frequency component with a reduced power loss.
In order to achieve the above-mentioned object, there is provided according to the present invention a pulse signal transmitting circuit, comprising:
a transformer having a primary winding and a secondary winding, the secondary winding being connected to a transmission cable;
first and second switching elements connected to the primary winding of the transformer, a voltage output from an external power source being input to the primary winding of the transformer when at least one of the first and second switching elements is closed so that a pulse voltage signal is input to the transformer; and
a booster power supply circuit provided between the external power source and a middle point of the primary winding of the transformer, the booster power supply circuit superimposing a boost voltage onto the voltage input to the primary winding of the transformer so that a high-frequency component of the pulse voltage signal input to the primary winding of the transformer is enhanced.
According to the above-mentioned invention, the booster power supply source generates a boost voltage and superimposes or adds the boost voltage to a voltage supplied from the power source. Accordingly, when one of the first and second switching elements is closed and the voltage of the power source is supplied to the transformer, the boost voltage is superimposed or added onto the voltage supplied to the transformer. Since the boost voltage is generated so as to enhance a high-frequency component of the pulse voltage signal to be input to the primary winding of the transformer, a pulse voltage signal generated in the secondary winding of the transformer also has an enhanced high-frequency component. Thus, the pulse signal transmitting circuit according to the present invention can generate a pulse voltage signal having an enhanced high-frequency component without causing power loss whereas the conventional apparatus generates a pulse voltage signal having an enhanced high-frequency component by cutting off a low-frequency component.
In the above-mentioned invention, the booster power supply circuit may store energy when the first and second switching elements are opened, the booster power supply circuit discharging the stored energy when at least one of the first and second switching elements is closed so as to enhance the high-frequency component of the pulse voltage signal supplied to the primary winding of the transformer.
According to this invention, the booster power supply circuit stores energy during a period in which the voltage of the power source is not supplied to either the first switching element or the second switching element. Thus, energy of the power source can be efficiently used.
In one embodiment of the present invention, the booster power supply circuit may comprise a coil having a predetermined inductance. The coil generates a reverse electromotive force when the pulse voltage signal is input to the transformer, and a voltage caused by the reverse electromotive force is superimposed onto the voltage supplied to the transformer.
Additionally, the pulse signal transmitting circuit according to the present invention may further comprise a third switching element connected in series with the coil so that an inverted signal of a sum of a first pulse signal supplied to the first switching element to close the first switching element and a second pulse signal supplied to the second switching element to close the second switching element is supplied to a control terminal of the third switching element,
wherein the third switching element is closed when the first and second switching elements are open so that a current flows to the coil through the third switching element; and
the third switching element is opened when at least one of the first and second switching elements is closed so that the current flowing to the coil is interrupted, and, thereby, a reverse electromotive force is generated by the coil and the voltage corresponding to the reverse electromotive force is superimposed on the voltage from the external power source.
According to this invention, the current to be supplied to the booster power supply circuit can be controlled by merely adding the third switching element and an inverter which inverts the sum of the first pulse signal and the second pulse signal. Thus, the booster power supply circuit can be achieved with a simple structure.
In one embodiment of the present invention, the booster power supply circuit may comprise a plurality of coils each of which has a predetermined inductance, and the pulse signal transmitting circuit may further comprise a booster selecting circuit which selects at least one of the plurality of coils according to a transmission characteristic and/or a length of the transmission cable.
Accordingly, an appropriate amount of boost can be obtained by selecting one or more coils from among the plurality of coils in accordance with a transmission characteristic and/or a length of the transmission cable to be used.
In one embodiment of the present invention, the booster selecting circuit may comprise a plurality of fourth switching elements each of which is connected in series with a respective one of the plurality of coils so that a selection of the coils is performed by operations of the switching elements.
Additionally, the pulse signal transmitting circuit according to this invention may further comprise a third switching element connected in series with each of the coils so that an inverted signal of a sum of a first pulse signal supplied to the first switching element to close the first switching element and a second signal supplied to the second switching element to close the second switching element is supplied to a control terminal of the third switching element,
wherein the third switching element is closed when the first and second switching elements are opened so that a current flows to at least one of the coils through the third switching element; and
the third switching element is opened when at least one of the first and second switching elements is closed so that the current flowing to at least one of the coils is interrupted, and, thereby, a reverse electromotive force is generated by the at least one of the coils and the voltage corresponding to the reverse electromotive force is superimposed on the voltage from the external power source.
According to this invention, the current supplied to the booster power supply circuit can be controlled by merely adding the third switching element and an inverter which inverts the sum of the first pulse signal and the second pulse signal. Thus, the booster power supply circuit can be achieved with a simple structure.
Additionally, in the pulse signal transmitting circuit according to the present invention, a first unipolar pulse signal may be supplied to a control terminal of the first switching element to close the first switching element, and a second unipolar pulse signal may be supplied to a control terminal of the switching element to close the second switching element so as to output a bipolar pulse signal from the secondary winding of the transformer.
In one embodiment of the present invention, the booster selecting circuit may comprise mechanical switches or electronic switches.
Additionally, there is provided according to another aspect of the present invention a subscriber""s line terminal apparatus having the above-mentioned pulse signal transmitting circuit.
The subscriber""s line terminal apparatus according to the present invention may have a plurality of channels, and the pulse signal transmitting circuit may be provided to each of the plurality of channels. In such a structure, the booster selecting circuit may be shared with a plurality of pulse signal transmitting circuits which results in a simple structure of the subscriber""s line terminal apparatus.
Other objects, features and advantages of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings.